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Calibrating the servos.

A project log for DIY hobby servos quadruped robot

Cheap 3D printed 4 legged robot, that almost looks like boston dynamics spot but moves like a newborn.

Miguel Ayuso ParrillaMiguel Ayuso Parrilla 08/01/2020 at 09:150 Comments

First of all, you will need to plug all servos to their proper pin, in my case here they are (you can change this for your how setup):

void connectServos() {
                      //FR
  Servos[0].attach(40); //coxa
  Servos[1].attach(38); //femur
  Servos[2].attach(36); //tibia
                      //FL
  Servos[3].attach(42); //coxa
  Servos[4].attach(44); //femur
  Servos[5].attach(46); //tibia
                     //BR
  Servos[6].attach(34); //coxa
  Servos[7].attach(32); //femur
  Servos[8].attach(30); //tibia
                     //BL
  Servos[9].attach(48); //coxa
  Servos[10].attach(50); //femur
  Servos[11].attach(52); //tibia
}

As soon as the servos are corretly attached, what i made in order to put the servos on its correct position is just to dissasembly the legs, i mean, separate the servo output from the its horn, in order to let the servo move freely. This way you can power the servos, let them go to the zero position and the reassembly.

I wrote a small sketch in order to put the servos on their zero position, you can find it at servo_calibration.py file, this way you can attatch the servo very close to their zero position.

Then, a more precise calibration is needed, you will need to calibrate the offset of each servo, those offset are defined at angleToPulse.py file. This file only contains the line defining the pulse corresponding to each angle for every servo.

Being a line in the form: y = Ax + B. The offset of each servo is defined by B, which mean the pulse at the zero angle.

If you are using different servos, the slope of the line (A) will be different so you will need to identify it. What i did, is just to take 4-5 meassurement of pulse at different angles and put them on an excel where i calculate the adjucted line from those points.

def convert(FR_angles, FL_angles, BR_angles, BL_angles):
    pulse = np.empty([12])
    #FR
    pulse[0] = int(-10.822 * np.rad2deg(-FR_angles[0])) + 950
    pulse[1] = int(-10.822 * np.rad2deg(FR_angles[1])) + 2280
    pulse[2] = int(10.822 * (np.rad2deg(FR_angles[2]) + 90)) + 1000
    #FL
    pulse[3] = int(10.822 * np.rad2deg(FL_angles[0])) + 1020 
    pulse[4] = int(10.822 * np.rad2deg(FL_angles[1])) + 570
    pulse[5] = int(-10.822 * (np.rad2deg(FL_angles[2]) + 90)) + 1150
    #BR
    pulse[6] = int(10.822 * np.rad2deg(-BR_angles[0])) + 1060 
    pulse[7] = int(-10.822 * np.rad2deg(BR_angles[1])) + 2335 
    pulse[8] = int(10.822 * (np.rad2deg(BR_angles[2]) + 90)) + 1200
    #BL
    pulse[9] = int(-10.822 * np.rad2deg(BL_angles[0])) + 890
    pulse[10] = int(10.822 * np.rad2deg(BL_angles[1])) + 710
    pulse[11] = int(-10.822 * (np.rad2deg(BL_angles[2]) + 90)) + 1050
    return pulse

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